Add command to start a new session without exiting

Feature Request

Add a slash command (e.g., /new-session) that starts a fresh session without having to exit and restart claude.

Motivation

Currently, /clear, /reset, and /new all clear the conversation history but keep the same session. This means session-level state like cumulative cost and token counts persist.

The only way to get a truly fresh session is to /exit and run claude again, which is inconvenient — especially in workflows where you want to start a new task but stay in the same terminal (e.g., tmux setups with statusline integrations that display per-session costs).

Proposed Behavior

A new command that:

• Creates a new session ID
• Resets cumulative cost and token counters
• Clears conversation history
• Stays in the same claude process

This would be equivalent to exiting and restarting, but without the overhead of spawning a new process.